patch 8.2.1576: Vim9: index() does not take "true" as argument

Problem:    Vim9: index() does not take "true" as argument.
Solution:   Use tv_get_bool_chk(). (closes #6823)
diff --git a/src/version.c b/src/version.c
index 2bf4914..388be57 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1576,
+/**/
     1575,
 /**/
     1574,